Urban-rural inequality in lung cancer risk from indoor air pollution: Prolonged indoor time amplifies the risks of solid fuel use

Indoor air pollution, particularly from solid fuel use, poses serious health risks. While some studies have examined additive interactions between environmental factors and genetic susceptibility, few have assessed such synergy using indoor time duration as a behaviorally grounded potential proxy of cumulative exposure, limiting the development of effective management strategies and policies. This matched case-control study investigated 286 non-smoking participants in China, emphasizing urban-rural disparities. A significant dose-response relationship between indoor time and lung cancer risk was identified, with odds ratios (ORs) of 1.90 (95 % CI: 1.04, 3.48) for 21-23 h/day indoors and 3.84 (95 % CI: 1.63, 9.05) for >23 h/day. This association was significant in rural populations but not in urban ones. Additive interactions between indoor time and genetic susceptibility were observed in the overall population [relative excess risk due to interaction (RERI) = 4.32, attributable proportion (AP) = 0.61, synergy index (S) = 2.92], rural residents (RERI = 30.10, AP = 0.84, S = 7.63), and solid fuel users (RERI = 20.90, AP = 0.70, S = 3.47), but not in urban populations or non-users. Rural residents, with 98 % relying on solid fuels compared to 20 % in urban areas, faced heightened risks as prolonged indoor time amplified the harmful effects of solid fuel combustion. These findings not only underscore urban-rural inequalities in lung cancer risk, but also offer novel perspectives for exposure assessment and vulnerability-informed policymaking.